The present invention relates to a fuel pump assembly, having a low-pressure region with at least one low-pressure pump and a high-pressure region with at least one regulated-quantity high-pressure pump, which pumps a quantity of fuel required to compensate for the quantitative balance in the high-pressure region, the quantity regulation being effected via a metering unit which is disposed between the low-pressure pump and the high-pressure pump.
Such fuel pump assemblies have long been known in the prior art. They are used predominantly in automotive engineering for delivering fuel to direct-injected engines. By regulating the quantity of fuel furnished by the high-pressure pump, greater efficiency of the fuel pump assembly is obtained and the operating temperature is reduced.
Because of the common rail injection technique that has come into use in recent years, such fuel pump assemblies have become widespread. In the common rail technique, a high-pressure fuel pump, a distributor line, and electromagnetic injection valves replace the conventional fuel injection pump and injection valves. Thus not only can the injection pump be varied entirely as needed between 250 and 1600 bar, but the instant and course of injection can be varied as desired by the specification of the electronic engine controller. The common rail injection technique provides for a reduction in fuel consumption and pollutant emissions as well as noise abatement, while at the same time providing better power.
The fuel pump assemblies known from the prior art have the disadvantage, however, that at certain operating points, and particularly in so-called zero pumping, when the high-pressure pump requires no fuel quantity and the metering unit is closed, a slight unintended pumping can still occur. Depending on how the metering unit functions, the unintended pumping is caused for instance by leakage or measurement errors on the part of the metering unit and can hardly be avoided despite major technological efforts to counteract it.
The unintentionally pumped fuel quantity irritates the high-pressure pump in the high-pressure region of the fuel pump assembly and is therefore, in the prior art, drawn from the high-pressure region again by suitable means, such as a pressure regulating valve. However, such pressure regulating valves are complicated in design, expensive to procure, and above all subject to major wear. As a consequence, the pressure regulating valves fail unpredictably, so that the availability of the entire fuel pump assembly, of the kind known in the prior art, is often unable to meet the high demands made in modern engine construction.